1. Field of the Invention
This invention relates to CDMA chip synchronization circuits which are provided in CDMA receivers to perform synchronous detection with respect to receiving timing of radio frequency signals. This application is based on patent application No. Hei 8-333393 filed in Japan, the content of which is incorporated herein by reference.
2. Prior Art
The CDMA chip synchronization circuit (where `CDMA` stands for `Code Division Multiple Access`) is provided in the receiver of the mobile communications system, particularly in the receiver of the automobile phone and portable phone system (which will be simply called "cellular system") using the DS-CDMA method (where `DS-CDMA` stands for `Direct Spread CDMA`). In other words, the CDMA chip synchronization circuit is used to detect the receiving timing at the base station receiver.
Examples of the spread spectrum communications are disclosed by the papers of Japanese Patent Laid-Open Publication Nos. 4-347944 and 6-284111 both of which relate to the synchronization device for the spread spectrum communications equipment as well as Japanese Patent Publication No. 2-39139 which relates to the receiver of the spread spectrum communications method, for example. In addition, other information regarding the disclosure of the spread spectrum communications is disclosed in, for example, Chapter 6 and Chapter 7 of the paper entitled "TIA/EIA INTERIM STANDARD (TIA/EIA/IS-95-A) Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System PN-3421 (to be published as IS-95-A)" which is published by the Telecommunication Industry Association (TIA) on May of 1994 as well as FIG. 3.1, FIG. 3.2, FIG. 3.6 and pages 39-66 of Chapter 3 of the paper entitled "Principles of Spread Spectrum Communication" which is written by Dr. Andrew J. Viterbi and published in May of 1995 by Addison-Wesley Publishing Company.
Among the conventional mobile communications systems, the so-called North American standard method (i.e., TIA IS95) is known as the digital cellular system using the CDMA method. In the standard specification of TIA/EIA/IS-95-A, Chapter 6 describes operations required for the mobile station, while Chapter 7 describes operations required for the base station. However, the above standard specification merely provides standardization for the radio interface. For this reason, the standard specification describes the modulation method and signal formats but fails to describe the concrete receiving method.
The forward link of IS-95-A (which is used to perform transmission from the base station to the mobile station) performs transmission of pilot channels (PLCH) in addition to transmission of traffic channels (TCH). Herein, the traffic channels are provided for multiple users subjected to modulation by information, whilst the pilot channels are not subjected to modulation by information. Further, the transmission of the pilot channels is performed using relatively intense electric power. Using the pilot channels, the mobile station is capable of determining the optimum receiving timing. So, the mobile station does not suffer as much from the problem that the receiving timing should be determined under low Eb/No (where `Eb` denotes energy of receiving signals per 1 bit of information; and `No` denotes electric power density of noise and interference signal per 1 Hz). However, transmission of the pilot channels using the intense electric power results in reduction of a number of traffic channels which are used for actual transmission of information. This causes another problem that a number of users per one base station should be reduced.
On the other hand, no common pilot channels exist in the reverse link of IS-95-A (which is used to perform transmission from the mobile station to the base station). So, the reverse link employs the modulation method corresponding to the 64-ary orthogonal code modulation combined with the quadruple direct spread. Using the 64-ary orthogonal codes, as compared with the BPSK and QPSK (where `BPSK` stands for `Binary Phase-Shift Keying` and `QPSK` stands for `Quaternary PSK`), it is possible to provide a variety of advantages:
It is possible to increase electric power per one symbol and it is possible to reduce deterioration for synchronous detection even if asynchronous detection is employed.
However, the above has problems relating to receiving method.
Main elements of the IS-95-A are determined such that the chip rate is set at 1.2288 Mcps, the bit rate is at 9.6 kbps and the spread rate of the direct spread is at 128. According to the above, the chip rate is relatively low speed (because of the narrow-band CDMA), wherein as compared with instantaneous variations of the propagation delay, the chip period is relatively long. For this reason, the amount of deterioration in receiving characteristics is small even if characteristics of the receiving timing detecting circuit are somewhat loose. However, to perform high-speed data communications with respect to voices and other information, it is necessary to increase the bit rate and chip rate by a certain factor of multiplication which ranges between 5 and 10; in other words, it is necessary to provide the wide band CDMA. In that case, other problems occur which the IS-95-A cannot expect. In the case of the chip rate of 10 Mpcs, for example, if the propagation path differs by 30 m, the receiving timing deviates from the original timing with respect to one chip only. So, it is not possible to receive signals with the original timing. In addition, a plurality of multipaths overlap with each other within a range of delay times corresponding to multiple chips. In that case, there is a problem that positions of peaks cannot be clearly defined.
A conventional example of the receiving timing detection method (or chip synchronization method) is taught by the paper entitled "Principles of Spread Spectrum Communication" which is written by Dr. Andrew J.
Viterbi and published May 1995 by Addison-Wesley Publishing Company. The operation to capture the timing of signals which are spread by spread codes corresponding to pseudo-random codes is performed in two stages of processing. That is, the method performs initial synchronous capture (or initial synchronous search) and synchronous tracking.
The method of the initial synchronous search is explained in the fourth paragraph of Chapter 3 of the above paper. According to this method, until the correlation electric power exceeds a certain threshold value, the search is performed in a serial manner with shifting the receiving timing by a half chip space.
The synchronous tracking corresponds to the method of so-called "early-late gate" or "delay lock loop (DLL)". This method calculates first correlation electric power corresponding to the early timing which is earlier than the reference timing by the delay time .DELTA.t for the receiving and second correlation electric power corresponding to the late timing which is later by .DELTA.t. Then, the method performs fine timing adjustment in such a way that a difference between the first electric power and second electric power becomes zero.
Meanwhile, Japanese Patent Laid-Open Publication No. 4-347944 discloses some improvements to the method of the initial synchronous search and synchronous tracking. Particularly, the paper discloses the method regarding the commonality of circuits as well as the method to add tracking function to the multipath propagation path. However, the basic operation of the above method is identical to that of the aforementioned paper written by Dr. Andrew J. Viterbi. In addition, this method is not capable of solving the aforementioned problems in the wide band CDMA.
Japanese Patent Publication No. 2-39139 describes a method to search a new path wherein operation of the sliding correlator is not made only in the initial synchronous search but is made normally. Similar description is found in Patent Laid-Open Publication No. 6-284111. According to the above method which is designed to perform searching of new paths normally, it is possible to shorten the instantaneous break time in communications. However, it cannot be said that the method is capable of detecting peak positions accurately with a short time.
In short, the mobile communications system using the CDMA method performs receiving of so-called multipath signals, wherein the system should perform matching of timing with respect to each of signals. Herein, the multipath signals are transmitted to the system via multiple propagation paths which differ from each other in propagation time due to the reflection by buildings and mountains, for example. To achieve effective usage of frequencies in communications, each channel should have a capability of receiving signals under the very low Eb/No environment.
Particularly, in case of the wide band CDMA method whose chip rate is 10 Mcps or so, the receiving timing is shifted by one chip when the propagation distance changes by 30 m. Such a shift makes the receiving operation impossible. A difference of propagation delay corresponding to a difference of propagation distance of 30 m or so easily occurs when a small variation occurs in the propagation path even if the distance between the base station and mobile station is unchanged. In other words, there frequently occurs a phenomenon that the receiving is performed with respect to the multipaths which overlap with each other in a range of multiple chips, and variations simultaneously occur with respect to receiving paths (i.e., emergence and vanishing of new paths).
Conventionally, the DLL technology is used for the synchronous tracking of the receiving timing. This technology is effective under prescribed conditions that each multipath has a separate peak and the propagation delay time continuously and gradually changes. In contrast, the wide band CDMA works under the condition where the receiving is performed with respect to the mutipaths in an overlap manner and the delay time discontinuously changes. So, the wide band CDMA suffers from a problem due to its inability to perform of the tracking.